Classification of Thunderstorms

There are three main conditions to consider in order to have a thunderstorm. These are adequate moisture, an unstable mass, and the lifting force of heat. Let's look first at the initial developing stage of a thunderstorm.

The first step is to have an uplifting of moist airmasses into the higher levels of the atmosphere. These masses require input from the sun onto the ground. The ground radiates heat that is lifted up to higher elevations and toward a mass of cooler air. Cumulus clouds are in this first stage, showing up because of condensation of water vapor. Remember that condensation gives off heat by itself so that the air is lifted further through convective processes. When the warmed air leaves the lower aspect of the clouds, low air pressure is left behind. Huge amounts of moisture enter the atmosphere.

After this, the thunderstorm is considered mature. Anvil clouds can be seen and warm air rises until it reaches the tropopause. Cumulus clouds become cumulonimbus clouds and air is forced to spread below the tropopause out from the center. The raindrops get much heavier and often freeze into ice particles. These will fall and melt to become rain. Prolonged updrafts will lead to ice so large that hail results. The coexistence of updrafts and rain-drenched downdrafts mean the thunderstorm is mature. Inside the clouds is turbulence and the chance for wind, lightning, and tornadoes.

The storm may simply rain out as long as there is little wind shear. The dissipation starts when the downdrafts exceed the updrafts. This can happen quickly unless there is a supercell. The downdraft will push through the clouds and often hit the ground. With nowhere to go, the air spreads out. You will call this a downburst. Such a strong cold downburst blocks any updraft from occurring. The thunderstorm can't sustain itself. These downbursts are potentially damaging to aircraft flying within them. The craft can lose their lift or can be damaged in windshear.

CLASSIFICATION OF THUNDERSTORMS

Thunderstorms come in four varieties. These are called single-cell storms, multi-cell storms, squall lines, and supercells. The simplest among these are single-cell storms,

having very little vertical windshear associated with them. These last less than 30 minutes.

The other types of storms are more organized and last longer, with a great deal of windshear in excess of 13 meters per second. There are strong updrafts and severe weather phenomena like hail and extreme winds. Supercells are particularly strong and linked to hail, wind, and tornadoes. There is a great deal of energy in these types of storms.

Single-celled storms have one major updraft. These are called air-mass thunderstorms, typically seen on summer days. They can come after a cold front has come in from the ocean in wintertime. By definition, a cell is defined as a single updraft. Some of these are called pulse severe storms if they have severe weather and short durations associated with them.

Multi-cell clusters involve more than one updraft; there is a mature thunderstorm in the middle with dissipating storms downwind of the main storm. Clusters can organize into squall lines. Each cell may last only 20 minutes, but the clusters altogether last several hours at a time. Strong cold fronts and mountain ranges may help form these larger clusters, capable of moderate hail, flooding, and minor tornadoes.

Squall lines are linear and consist of multiple cells that have lined up along or ahead of a cold front. Expect heavy precipitation, lightning, hail, straight-line winds, waterspouts, and tornadoes. Where you see a bow echo on Doppler radar is where the straight-line winds emerge. Bow echoes known as derechos can move very quickly.

Supercells are very large storms associated with windshear. Windshear is associated with changes in windspeed and direction with the height of the storm. There will be many different updrafts and downdrafts with these storms. These create the strongest storms where the winds can break through the tropopause to enter the stratosphere. The storms may be up to 15 miles in width. Surprisingly, up to 10 percent of these do not have severe weather as part of them, although most do. There can be rotating updrafts in them, called mesocyclones. Most tornadoes come from these kinds of thunderstorms.

You would designate a thunderstorm as severe if there are winds greater than 58 miles per hour or hail up to one inch in diameter. If you see tornadoes, you would upgrade the